There is known a switching device which is an active switching element built around an operational amplifier having two key elements placed in series in its negative feedback circuit. When the key elements are energized, the negative feedback circuit of the operational amplifier is closed, and the transition resistance between the input and output terminals of the device is reduced to a small value (cf. S. Marjanovic, D. R. Noaks, "A High Speed, High Accuracy Digitally-Set Potentiometer," Radio and Electron, Eng., 1969, vol. 38, No. 6, pp. 345-351).
The device under review has a low reliability because, when the key elements make or break the circuit, the operational amplifier is not protected from overvoltages in its input circuit. Apart from being the cause of the low reliability of the switching device under review, this disadvantage accounts for a limited sphere of application of switching devices incorporating active switching elements.
It is an object of the present invention to improve the reliability of the switching device.